A variety of methods have been employed for optically pumping solid-state lasers, such as neodymium-doped lithium yttrium fluoride (Nd:YLF) or neodymium-doped yttrium aluminum garnet (Nd:YAG). A common method is to use an arc lamp or other similar light source to excite a laser rod. The light source and laser rod are positioned within and at different foci of a highly reflective housing of elliptical cross-section. This method typically requires relatively large diameter laser rods to efficiently absorb enough of the pumping light emitted by the light source to allow solid-state laser operation. Another limitation of this pumping method is the relative inefficiency caused by poor overlap of the optical emission spectrum of the pumping light source with the absorption bandwidth of the solid-state lasants.
This method typically entails the use of arc-pumped, solid-state lasers, which commonly require water cooling systems that place a severe cumbrance on clean room environments typically preferred for link processing of dynamic random access memory devices performed by, for example, arc-pumped, Q-switched, Nd:YAG lasers.
There are several different methods for diode-pumping solid-state lasers. In U.S. Pat. No. 3,982,201, Rosenkrantz et al. describe a solid-state laser that is pumped by single diode lasers or arrays of diode lasers to which the solid-state laser rod is directly end-coupled. Because the output wavelength of the diode laser array is a function of its temperature, the diode lasers are operated in a pulsed mode at a low duty cycle to maintain the array at a sufficiently stable temperature so that its output wavelength remains matched to the absorption bandwidth of the solid-state laser rod. The output power characteristics of this laser system are limited by the relatively inefficient match between the output of the diode lasers and the mode volume of the solid-state laser rod.
In "Efficient LiNdP.sub.4 O.sub.12 Lasers Pumped with a Laser Diode," Applied Optics, vol. 18, No. 23 (Dec. 1, 1979), Kubodera and Otsuka describe the well-known practice of collecting the output light of a diode laser and focusing its expanded output light using conventional lenses, such as two microscope condenser lenses. This method is particularly well suited for applications where emitter width and divergence of the diode laser are small. However, as the emitter dimensions and beam divergence increase, it becomes increasingly difficult to efficiently collect the output beam with collection lens or lenses. It also becomes more difficult to focus the expanded beam into the solid-state lasant crystal with sufficient depth of focus to allow efficient overlap of the pump beam throughout the resonator mode volume within the lasant.
In U.S. Pat. No. 4,710,940, Sipes, Jr. describes a Nd:YAG solid-state laser that is end-pumped by a diode laser array or by two diode laser arrays that have been combined by use of polarizing beam-splitting cubes. Sipes, Jr., cites the analysis of D.G. Hall in "Optimum Mode Size Criteria for Low Gain Lasers," Applied Optics, 1579-1583, vol. 20, (May 1, 1981), to suggest that the "pump profile shape does not matter much as long as all the pump light falls within the resonator mode." Sipes, Jr., notes, however, that Hall's analysis does not account for the divergence properties of Gaussian beams, so Sipes, Jr., suggests that, if required, the cross-section of the pump beam could be modified by use of a cylindrical lens.
In U.S. Pat. No. 4,761,786, Baer describes a Q-switched, solid-state laser that is end-pumped by a diode laser or diode laser array. The output light from the pump source is collected by a collimating lens and directed by a focusing lens to end-pump the laser rod. Baer notes that "other lenses to correct astigmatism may be placed between the collimating lens and focusing lens." Baer also describes an alternate embodiment that employs a remotely positioned diode laser pumping source coupled through an optical fiber, the output of which is focused by a lens into the laser rod.
In U.S. Pat. No. 4,763,975, Scifres et al. describe two optical systems that produce bright light output for a variety of applications, including pumping a solid-state laser such as a Nd:YAG. Scifres et al. describe an optical system that employs a plurality of diode lasers, each of which is coupled into one of a plurality of fiber-optic waveguides. The waveguides are arranged to form a bundle and the light from the diode laser sources is emitted at the output end of the bundle. Optics, such as a lens, may be used to focus the light into a solid-state laser medium. Alternatively, the fiber bundle may be "butt"-coupled to the laser rod. (Butt coupled means end-coupled at a position very close to or in contact with the laser rod.)
Scifres et al. describe another optical system that employs a diode laser bar, broad-area laser, or other elongated source to pump a solid-state laser. The diode laser bar light output is coupled into a fiber-optic waveguide having an input end that has been squashed to be elongated and thereby have core dimensions and lateral and transverse numerical apertures that correspond respectively to those of emission dimension and lateral and transverse divergence angles of the diode laser bar. The output light from the fiber-optic waveguide is either focused by a lens into the end of the solid-state laser rod or butt-coupled to the rod. Scifres et al. state that either end of the fiber-optic waveguide can be curved. Although these methods attempt to match the output light from the fiber-optic waveguide to the resonant cavity mode of the solid-state laser, they are limited in efficiency by the numerical aperture of the sources that can be effectively collected and guided by the fiber-optic waveguides.
Certain methods are known for efficiently coupling the output of high-power diode lasers into solid-state lasants. High-power diode lasers are necessarily broad-area devices or arrays of narrow-width diode lasers because the potential for catastrophic optical damage to the mirrors dictates the optical outputs be limited typically to 10 to 20 mW per micron of emission stripe width. Typical high-power diode lasers used to pump solid-state lasants include aluminum gallium arsenide (AlGaAs) diode lasers. Examples of such laser diodes include Model No. SDL-2480-P1 with continuous wave (CW) output power of 3.0 watts (W) and an emission width of 500 .mu.m; Model No. SDL-2462-P1 with CW output power of 1.0 W 35 and an emission width of 200 .mu.m; and Model No. SDL-2432-P1 with CW output power of 0.5 W and an emission width of 100 .mu.m, all of which are manufactured by Spectra Diode Labs, 80 Rose Orchard Way, San Jose, California. Use of AlGaAs semiconductor diode lasers to optically pump solid-state lasers has led to development of compact, solid-state lasers.
Broad-area lasers are described by G.H.B. Thompson in "A Theory for Filamentation in Semiconductor Lasers", Optoelectronics, 257-310, vol. 4, (1972) and by P.A. Kirkby, et al. in "Observations of Self-Focusing in Stripe Geometry Semiconductor Lasers and Development of a Comprehensive Model of Their Operation," IEEE Journal of Ouantum Electronics, 705-719, vol. QE-13 (1977). Such broad-area lasers (emission width of typically greater than 5 .mu.m) typically exhibit a filamentary structure in their optical near-field patterns. The filament structures arise from a nonlinear interaction between the carriers and the optical field in the active area of the laser. The process of stimulated emission effectively reduces the gain profile within the active area and results in an increase in the refractive index in the portion of the active area contributing most strongly to the optical mode. This region of increased refractive index is bounded by regions of the active area which do not contribute so strongly to the optical mode and are characterized by smaller refractive index values. This lateral variation in refractive index in a local region within the active area of the diode laser can form a local lateral index guide.
When the active area is broader than about 5-10 .mu.m, as is the case in typical high-power diodes used for solid-state laser pumping, several, or in some cases, many such index-guided regions may form. Stimulated emission within each such lateral index-guided region within the active area may occur in the form of a filament that is only partly spatially coherent or is spatially incoherent with respect to neighboring filaments. This filamentation phenomena is, therefore, a fundamental source of lateral spatial incoherence in high-power laser diodes and, consequently, places limits on the optical brightness obtainable from such devices.
Concurrently filed U.S. patent application of Baird, DeFreez, and Sun for Method and Apparatus for Generating and Employing a High Density of Excited Ions in a Lasant, which is assigned to assignee of the present application, describes a method for employing a high-power diode laser to longitudinally optically pump the mode volume of a solid-state lasant. The high-power diode laser in the preferred embodiment described by Baird et al. is of a type that is typically gain-guided in the lateral plane of the device and index-guided in the transverse plane. Accordingly, the laser diode is typically spatially incoherent in the lateral plane, thus limiting its optical brightness. Baird et al. also describe a method of employing a nonimaging concentrator to efficiently collect optical output from such a high-power diode laser and couple it into the mode volume of a solid-state lasant.
Although these methods have with varying degrees of efficiency been used to optically pump solid-state laser mode volumes and been used to produce useful solid-state laser output at a variety of emission wavelengths, improved methods for coupling the optical output from diode lasers into solid-state lasants are highly desirable. Such methods would be very useful in diode pumping of the new chromium-doped solid-state laser materials such as chromium:lithium calcium aluminum fluoride (Cr:LiCAlF) and chromium:lithium strontium aluminum fluoride (Cr:LiSAlF). These solid-state laser materials are described by S.A. Payne, et al., in "LiCaAlF.sub.6 :Cr.sup.3+ : A Promising New Solid-State Laser Material," IEEE Journal of Ouantum Electronics, 2243-2252, vol. 24, No. 11, (November 1988); S.A. Payne, et al., in "Laser Performance of LiSrAlF.sub.6 :Cr.sup.3+," in Journal of Aoolied Physics, 1051-1055, vol. 66, No. 3; and by S.A. Payne et al. in U.S. Pat. No. 4,811,349.
These inhomogenously broadened materials can be optically pumped by aluminum gallium indium phosphide (AlGaInP) laser diodes as described by Scheps, et al., in "Cr:LiCaAlF.sub.6 Laser Pumped by Visible Laser Diodes," IEEE Journal of Ouantum Electronics, 1968-1970, vol. 27, No. 8, (August 1991) and by Scheps, et al., in "Diode-Pumped Cr:LiSrAlF.sub.6 Laser," Optics Letters, 820-822, vol. 16, No. 11, (Jun. 1, 1991). However, the relatively low stimulated emission cross-section-fluorescence lifetime product of these materials implies a requirement for relatively large pump powers to obtain laser operation at threshold by pumping with such broad area, high-power diode lasers. This requirement results from the relatively large pumping beam radius inherent from the lateral spatial incoherence typical of such devices. The optical output of such a broad-area, high-power diode laser coupled via conventional methods into such an inhomogenously broadened material is, therefore, a relatively inefficient process.
A method for theoretically obtaining high-power, nearly diffraction-limited optical output from a high-power laser diode has recently been described by Tilton, . . . DeFreez, et al., in "High Power, Nearly Diffraction-Limited Output from a Semiconductor Laser with an Unstable Resonator," IEEE Journal of Ouantum Electronics, 2098-2108, vol. 27, No. 9, (September 1991). The high-power AlGaAs laser diode described therein demonstrates high power (greater than 1 watt from both facets) and nearly diffraction-limited optical output. The reference states that "[f]or many semiconductor laser applications, such as solid-state laser end pumping . . . , single-lobed, diffraction-limited beams of hundreds of millowatts are required." However, methods for coupling the bright optical output from an unstable resonator semiconductor laser (URSL) into a solid-state lasant have not heretofor been attempted. Furthermore, improvements in development of high-power URSL devices for use as very bright optical pumping sources for solid-state lasers are, therefore, also highly desirable.
Thus, improved methods for coupling the optical output of high-power diode lasers, especially those having improved lateral spatial coherence, into the mode volumes of a laser medium is highly desirable. Such methods for pumping Cr:LiCAlF and Cr:LiSAF to ultimately produce usable frequency doubled optical output in the 360-460 nm wavelength range are described in detail in concurrently filed U.S. patent application of Baird and DeFreez for High-Power, Compact, Diode-Pumped, Tunable Laser, which is assigned to assignee of the present application.